[Bug c++/80859] New: Performance Problems with OpenMP 4.5 support

["thorstenkurth at me dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80859

            Bug ID: 80859
           Summary: Performance Problems with OpenMP 4.5 support
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thorstenkurth at me dot com
  Target Milestone: ---

Dear Sir/Madam,

I am working on the Cori HPC system, a Cray XC-40 with intel Xeon Phi 7250. I
probably found a performance "bug" when using the OpenMP 4.5 target directives.
It seems to me that the GNU compiler generates unnecessary move and push
functions when a 

#pragma omp target region is present but no offloading is used.

I have attached a test case to illustrate that problem. Please compile the
nested_test_omp_4dot5.x in the directory (don't be confused by the name, I am
not using nested OpenMP here). Then go into the corresponding .cpp file and
comment out the target-related directives (target teams and distribute),
compile again and then compare the assembly code. The code with the target
directives has more pushes and moves than the one without. I think I also place
the output of that process in the directory already, the files ending in .as.

The performance overhead is marginal here but I am currently working on a
Department of Energy performance portability project and I am exploring the
usefulness of OpenMP 4.5. The code we retargeting is a Geometric Multigrid in
the BoxLiv/AMReX framework and there the overhead is significant. I could
observe as much as 10x slowdown accumulated throughout the app. This code is
bigger and thus I do not want to demonstrate that here but I could send you an
invitation to the github repo if requested. In my opinion, if no offloading is
used, the compiler should just ignore the target region statements and just
default to plain OpenMP. 

Please let me know what you think.

Best Regards
Thorsten Kurth
National Energy Research Scientific Computing Center
Lawrence Berkeley National Laboratory